1. Technical Field
The present invention relates to a method of lacquering the luminophore layer on the screen trough of a colour picture tube.
2. State of the Art
When lacquering the luminophore layer on the screen trough of a colour picture tube, it is customary to make use of an aqueous dispersion of a lacquer. Examples of suitable lacquers are given, among others, in U.S. Pat. No. 3,317,337 and DE-A-30 46 373 (U.S. Pat. No. 4,409,352). In practice there are essentially two methods for applying the dispersion. One of these, described--for example--in U.S. Pat. No. 3,317,337, consists of arranging the screen trough so that its luminophore surface points upwards, filling dispersion into it, and then causing it to revolve at a high speed so as to distribute the dispersion, eventually tipping the trough sideways to permit excess dispersion to drain off. A second known application method consists of spraying the dispersion upwards and against the screen, which for this purpose is set either vertically or pointing obliquely downwards. Following the spraying, the screen trough is set to rotate at speed so as to distribute the dispersion evenly over the luminophore layer and, at one and the same time, to throw off the excess dispersion.
With a view to making possible good wetting of the luminophore layer by the dispersion, it is common practice to precede the application of the dispersion to the luminophore by a prewetting step. Materials and methods suitable for this purpose are described, for example, in EP-B-149 838. Following the even distribution of the dispersion over the luminophore layer, the dispersion is fully dried and then heated to above the film formation temperature of the lacquer in order to produce a lacquer film. Subsequently--although this no longer forms part of the method of lacquering the luminophore layer--vaporized aluminium is deposited on the lacquer, so that the latter volatilizes and only an aluminium film is left as a reflection layer behind the luminophore layer.
Conventional methods of lacquering the luminophore layer on the screen trough of a colour picture tube therefore comprise the following steps:
the luminophore layer is prewetted, PA1 an aqueous lacquer dispersion is distributed evenly over the luminophore layer, and PA1 the dispersion is fully dried and heated to above the film formation temperature of the lacquer, thereby producing a lacquer film. PA1 the luminophore layer is first prewetted, PA1 a first aqueous dispersion is then applied to the prewetted luminophore layer, PA1 this first dispersion is fully dried and heated to above the film formation temperature of the lacquer, thereby producing a first lacquer film, PA1 a second aqueous dispersion is applied directly onto the first, the said second aqueous dispersion having exactly the same components as the first, where the concentration of the components in the second dispersion corresponds at least to a large extent to the concentration of the components in the first dispersion, and
Apart from these traditional methods with a single lacquering step, methods involving two lacquering steps are also known in literature. The previously mentioned U.S. Pat. No. 3,317,337, for example, describes a method according to which a 6% dispersion of a polyvinyl acetate in water is first applied to the luminophore layer. This first dispersion is then fully dried and heated to above the film formation temperature of the lacquer in order to produce a first lacquer film. The next step consists of applying a 10% dispersion of an acrylic resin in water, again followed by drying and heating to above the film formation temperature. Neither of the two lacquering steps is preceded by prewetting. Both layers are applied by spraying a puddle into the centre of the screen trough, which is then made to rotate.
An identical procedure, though using different aqueous dispersions, is described in DE-A-27 10 308. According to the examples given in the specification, the first dispersion contains either about 3% or about 6% of a copolymer of n-butyl acrylate and metacrylic acid, while the second dispersion contains about
15% of an acrylic resin.
Procedures using two layers of lacquer have not proved successful in practice largely in view of the fact that the first lacquer film was often damaged in the process of applying the second dispersion. No clear advantage ensued which could offset this drawback.
Notwithstanding the many known lacquering methods, all different from each other, there still remained the problem of so configuring a method of this type as to obtain better lacquering results , which ultimately means greater screen brightness.